58 AN INTRODUCTION TO ENTOMOLOGY 



and sometimes much broader, than the other segments. In such 

 cases this segment is frequently designated as the metatarsus (Fig. 

 67, C, m). 



In some insects the claws borne by the distal end of the tarsus are 

 outgrowths of a small terminal portion of the leg, the sixth segment 

 of the tarsus of some authors. This terminal part with its appendages 

 has received the name prcetarsus (De Meijere '01). As a rule the 

 prsetarsus is withdrawn into the fifth segment of the tarsus or is not 

 piesent as a distinct segment. 



On the ventral surface of the segments of the tarsus in many 

 insects are cushion-like structures; these are called pulvUli. The 

 cuticula of the pulvilli is traversed by nvimerous pores which open 

 either at the surface of the cuticula or through hollow hairs, the 

 tenent-hairs, and from which exudes an adhesive fluid that enables the 

 insect to walk on the lower surface of objects. 



With many insects (e. g. most Diptera) the distal segment of the 

 tarsus bears a pair of pulvilli, one beneath each claw. In such cases 

 there is frequently between these pulvilli a third single appendage of 

 similar structure; this is called the empoiium; writers on the Orthop- 

 tera commonly called the appendage between the claws the arolium. 

 In other insects the empodium is bristle-like or altogether wanting. 



In many insects the pulvillus of the distal segment of the tarsus 

 is a circular pad projecting between the tarsal claws. In many 

 descriptive works this is referred to as the pulvillus , even though the 

 other pulvilli are well-developed. The pulvilli are called the onychii 

 by some writers. 



The claws borne at the tip of the tarsus are termed the tarsal claws 

 or ungues; they vary much in form; they are usually two in nimiber, 

 but sometimes there is only one on each tarsus. 



The wings. — The wings of insects are typically two pairs of mem- 

 branous appendages, one pair borne by the mesothorax and one pair 

 by the metathorax; prothoracic wings are unknown in living insects 

 but they existed in certain paleozoic forms. 



Excepting in the subclass Apterygota which includes the 

 orders Thysanura and Collembola, wings are usually present in adult 

 insects. Their absence in the Apterygota is due to the fact that 

 they have not been evolved in this division of the class Hexapoda; 

 but when they are absent in adult members of the subclass Pterygo- 

 ta, which includes the other orders of insects, their absence is due 

 to a degradation, which has resulted in their loss. 



